1. Field of the Invention
Apparatus and methods consistent with the present invention relate to transmitting data more efficiently in network, and more particularly, to transmitting data between stations in a power line communication (PLC) network while preventing a hidden node problem.
2. Description of the Related Art
A PLC network technology method is one of a number of methods used to build a communication network for a home network. PLC network technology uses existing residential power lines. Accordingly, unlike Ethernet networks, PLC networks do not need additional wiring to be installed, and can be used in basements or areas that wireless networks cannot reach.
FIG. 1 is a schematic drawing illustrating data communication between a coordinator and stations in a PLC network. In FIG. 1, the dotted lined circle represents the communication range of a coordinator, the circle on the left represents the communication range of a station A, and the circle on the right represents the communication range of a station B.
Referring to FIG. 1, station A is outside the communication range of station B and station B is outside the communication range of station A. Accordingly, stations A and B are unable to detect each other's communication status. Thus, station B is a hidden node of station A, and station A is a hidden node of station B. Due to this fact, when stations A and B attempt to transmit data to the coordinator or other stations at the same time, the data transmissions interfere with one another. Hereinafter, an interference of data transmissions will be referred to as a “hidden node problem.”
When data transmissions from stations A and B, which have a hidden node relationship, collide with each other while being transmitted to the coordinator, the data is retransmitted based on an automatic repeat request (ARQ) mechanism. Consequently, data throughput deteriorates due to the data retransmission.
When a hidden node exists in a network, collision of data transmissions occurs very frequently. In same cases, 30 to 40% of data throughput deterioration is due to frequent collision of data transmissions.
Accordingly, an effective method for transmitting data is required to solve the hidden node problem.
FIG. 2 is a timing diagram illustrating a method of solving a hidden node problem by transmitting request to send/clear to send (RTS/CTS) commands to a PLC network before transmitting data during data communication between stations. The RTS/CTS commands are used to reserve a transmission medium in the PLC network before transmitting the actual data.
Before transmitting the data to a receiving station, a transmitting station transmits the RTS command to the PLC network. Upon receiving the RTS command, the receiving station transmits the CTS command to the PLC network. After transmitting the RTS command, the transmitting station transmits the actual data to the receiving station. Also, when data transmission is completed, the receiving station transmits an ACK command to the transmitting station and the PLC network to notify them of the completion of data transmission.
When other stations, besides the transmitting and receiving stations, receive the RTS/CTS commands, the other stations perform virtual carrier sense (VCS) until the data transmission has been completed in order to maintain data transmission standby status. In other words, stations in the PLC network, other than the transmitting and receiving stations, maintain standby status (referred to as “deferred access status”) and wait to transmit data after the completion of data transmission between the transmitting and receiving stations. Here, the VCS denotes standing by until the corresponding data transmission is completed.
Referring to FIG. 2, a deferred access period of a station that has received the RTS command begins from the point when the station receives the RTS command and continues to the point when the VCS of the station and an inter frame space (IFS) are complete. Here, the VCS of the station is performed from the point when the transmitting station transmits the RTS command to the station to the point when the receiving station transmits an acknowledgement (ACK) command to the station. After the deferred access period of the station, other stations, other than the transmitting and receiving stations which received the RTS command, can transmit data.
Referring to FIG. 2, deferred access period of a station that has received the CTS command begins from the point when the station receives the CTS command and continues to the point when the VCS of the station and an IFS are complete. Here, the VCS of the station is performed from the point when the transmitting station transmits the CTS command to the station to the point when the receiving station transmits the ACK command to the station. After the deferred access period of the station, other stations, other than the transmitting and receiving stations which received the CTS command, can transmit data.
As described above, by transmitting the RTS/CTS commands to the PLC network before the actual data transmission, the other stations, besides the transmitting and receiving stations, maintain a deferred access status, thereby preventing interference of data transmissions due to hidden nodes. Particularly when data throughput is not an important matter, using the RTS/CTS commands can be very useful in solving hidden node problems. However, using the RTS/CTS commands may come at the cost of data throughput in the PLC network, because the RTS/CTS commands may cause an overhead in the PLC network. Accordingly, if data throughput is important, using the RTS/CTS commands may be disadvantageous. Moreover, when data packet size is small, the data throughput deterioration caused by using the RTS/CTS commands may be larger than the data throughput deterioration caused by hidden node problems.
Accordingly, while transmitting data in the PLC network, the RTS/CTS commands coming at the cost of data throughput due to an overhead in the PLC network caused by the RTS/CTS commands should be prevented.